Lowering signal loss in an optical fiber is desirable because a signal can be transmitted a longer distance before signal amplification becomes necessary and/or at a reduced signal power. Of the silica-based class of optical fibers, pure silica-core optical fibers (i.e., without any more than trace amounts (less than about 0.3% by weight) of dopants in the core region) provide the lowest attenuation. Pure silica-core optical fibers can achieve an attenuation of 0.165 dB/km at 1550 nm in comparison with 0.19 dB/km at 1550 nm for a silica-core optical fiber doped with 3.2 mol % of GeO2.
To achieve the necessary waveguiding characteristic, the cladding in a pure silica-core optical fiber is desired to be at a lower index of refraction than the cladding in a doped silica-core optical fiber. The lower index of refraction is required to contain the light primarily in the core. One way of lowering the index of refraction of the cladding without adversely affecting attenuation of the fiber is by fabricating the cladding doped with fluorine. A pure silica-core fiber with a doped cladding of approximately 1.3% by weight of fluorine is sufficient to provide the necessary index of refraction difference between the core and cladding for single mode operation in the 1550 nm wavelength region. Unfortunately, a pure silica-core fiber with a halogen doped cladding exhibits greater stress or viscosity mismatch between the core and cladding than a GeO2 doped silica-core optical fiber. Stress between the core and cladding results in stress at different points along the fiber and/or induces index of refraction variation in the doped cladding.
Thus, there is a need for an optical fiber having attenuation comparable to that of at least a pure silica-core fiber and which provides the necessary waveguide function for signals in the infra-red wavelength region. There is a further need for fabricating adjacent sections of an optical fiber having reduced stress therebetween without requiring additional equipment and/or undue fabrication costs. There is a further need for fabricating adjacent sections of an optical fiber having reduced stress using a plasma chemical vapor deposition (PCVD) process.